Add support for JSON Schema const keyword

src/NJsonSchema.CodeGeneration.CSharp.Tests/DefaultPropertyTests.cs

@@ -200,5 +200,59 @@ public async Task When_generating_CSharp_code_then_default_value_of_array_of_arr
             await VerifyHelper.Verify(code);
             CSharpCompiler.AssertCompile(code);
         }
+
+        [Fact]
+        public async Task When_property_has_const_string_value_then_property_is_readonly_with_default_value()
+        {
+            // Arrange
+            var data = @"{
+                ""type"": ""object"",
+                ""properties"": {
+                    ""cmdType"": {
+                        ""const"": ""person""
+                    }
+                }
+            }";
+
+            var schema = await JsonSchema.FromJsonAsync(data);
+            var settings = new CSharpGeneratorSettings
+            {
+                ClassStyle = CSharpClassStyle.Poco,
+                Namespace = "ns"
+            };
+            var gen = new CSharpGenerator(schema, settings);
+            var output = gen.GenerateFile("MyClass");
+
+            // Assert
+            await VerifyHelper.Verify(output);
+            CSharpCompiler.AssertCompile(output);
+        }
+
+        [Fact]
+        public async Task When_property_has_const_integer_value_then_property_is_readonly_with_default_value()
+        {
+            // Arrange
+            var data = @"{
+                ""type"": ""object"",
+                ""properties"": {
+                    ""myNumber"": {
+                        ""const"": 42
+                    }
+                }
+            }";
+
+            var schema = await JsonSchema.FromJsonAsync(data);
+            var settings = new CSharpGeneratorSettings
+            {
+                ClassStyle = CSharpClassStyle.Poco,
+                Namespace = "ns"
+            };
+            var gen = new CSharpGenerator(schema, settings);
+            var output = gen.GenerateFile("MyClass");
+
+            // Assert
+            await VerifyHelper.Verify(output);
+            CSharpCompiler.AssertCompile(output);
+        }
     }
 }

src/NJsonSchema.CodeGeneration.CSharp.Tests/Snapshots/DefaultPropertyTests.When_property_has_const_integer_value_then_property_is_readonly_with_default_value.verified.txt

@@ -0,0 +1,27 @@
+//----------------------
+// <auto-generated>
+// </auto-generated>
+//----------------------
+
+
+namespace ns
+{
+    #pragma warning disable // Disable all warnings
+
+    public partial class MyClass
+    {
+
+        [Newtonsoft.Json.JsonProperty("myNumber", Required = Newtonsoft.Json.Required.DisallowNull, NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore)]
+        public int MyNumber { get; } = 42;
+
+        private System.Collections.Generic.IDictionary<string, object> _additionalProperties;
+
+        [Newtonsoft.Json.JsonExtensionData]
+        public System.Collections.Generic.IDictionary<string, object> AdditionalProperties
+        {
+            get { return _additionalProperties ?? (_additionalProperties = new System.Collections.Generic.Dictionary<string, object>()); }
+            set { _additionalProperties = value; }
+        }
+
+    }
+}

src/NJsonSchema.CodeGeneration.CSharp.Tests/Snapshots/DefaultPropertyTests.When_property_has_const_string_value_then_property_is_readonly_with_default_value.verified.txt

@@ -0,0 +1,27 @@
+//----------------------
+// <auto-generated>
+// </auto-generated>
+//----------------------
+
+
+namespace ns
+{
+    #pragma warning disable // Disable all warnings
+
+    public partial class MyClass
+    {
+
+        [Newtonsoft.Json.JsonProperty("cmdType", Required = Newtonsoft.Json.Required.DisallowNull, NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore)]
+        public string CmdType { get; } = "person";
+
+        private System.Collections.Generic.IDictionary<string, object> _additionalProperties;
+
+        [Newtonsoft.Json.JsonExtensionData]
+        public System.Collections.Generic.IDictionary<string, object> AdditionalProperties
+        {
+            get { return _additionalProperties ?? (_additionalProperties = new System.Collections.Generic.Dictionary<string, object>()); }
+            set { _additionalProperties = value; }
+        }
+
+    }
+}

src/NJsonSchema.CodeGeneration.CSharp/CSharpTypeResolver.cs

@@ -81,7 +81,8 @@ schema is JsonSchemaProperty property &&
                 schema.InheritedSchema == null && // not in inheritance hierarchy
                 schema.AllOf.Count == 0 &&
                 !Types.ContainsKey(schema) &&
-                !schema.HasReference)
+                !schema.HasReference &&
+                !schema.ActualTypeSchema.HasConstValue)
             {
                 return markAsNullableType ? Settings.AnyType + "?" : Settings.AnyType;
             }
@@ -94,6 +95,11 @@ schema is JsonSchemaProperty property &&
                     : JsonObjectType.String;
             }
 
+            if (type == JsonObjectType.None && schema.ActualTypeSchema.HasConstValue)
+            {
+                type = schema.ActualTypeSchema.ConstValueType;
+            }
+
             if (type.IsNumber())
             {
                 return ResolveNumber(schema.ActualTypeSchema, isNullable);

src/NJsonSchema.CodeGeneration.CSharp/Models/PropertyModel.cs

@@ -71,8 +71,9 @@ public PropertyModel(
 
         /// <summary>Gets a value indicating whether this is an array property which cannot be null.</summary>
         public bool HasSetter =>
-            _property.IsNullable(_settings.SchemaType) || (!_property.ActualTypeSchema.IsArray || !_settings.GenerateImmutableArrayProperties) &&
-                (!_property.ActualTypeSchema.IsDictionary || !_settings.GenerateImmutableDictionaryProperties);
+            !_property.ActualSchema.HasConstValue &&
+            (_property.IsNullable(_settings.SchemaType) || (!_property.ActualTypeSchema.IsArray || !_settings.GenerateImmutableArrayProperties) &&
+                (!_property.ActualTypeSchema.IsDictionary || !_settings.GenerateImmutableDictionaryProperties));
 
         /// <summary>Gets the json property required.</summary>
         public string JsonPropertyRequiredCode

src/NJsonSchema.CodeGeneration.TypeScript/TypeScriptTypeResolver.cs

@@ -89,7 +89,8 @@ private string Resolve(JsonSchema schema, string? typeNameHint, bool addInterfac
                 schema.InheritedSchema == null && // not in inheritance hierarchy
                 schema.AllOf.Count == 0 &&
                 !Types.ContainsKey(schema) &&
-                !schema.HasReference)
+                !schema.HasReference &&
+                !schema.ActualTypeSchema.HasConstValue)
             {
                 return "any";
             }
@@ -102,6 +103,11 @@ private string Resolve(JsonSchema schema, string? typeNameHint, bool addInterfac
                     JsonObjectType.String;
             }
 
+            if (schema.ActualTypeSchema.HasConstValue)
+            {
+                return ResolveConst(schema.ActualTypeSchema);
+            }
+
             if (type.IsNumber())
             {
                 return "number";
@@ -296,6 +302,22 @@ private static string ResolveInteger(JsonSchema schema, string? typeNameHint)
             return "number";
         }
 
+        private static string ResolveConst(JsonSchema schema)
+        {
+            var constType = schema.ConstValueType;
+            if (constType.IsBoolean())
+            {
+                return schema.Const!.ToString()!.ToLowerInvariant();
+            }
+
+            if (constType.IsInteger() || constType.IsNumber())
+            {
+                return ValueGeneratorBase.ConvertToNumberToStringCore(schema.Const!);
+            }
+
+            return ConversionUtilities.ConvertToStringLiteral(schema.Const!.ToString() ?? string.Empty, "\"", "\"");
+        }
+
         private string ResolveArrayOrTuple(JsonSchema schema, string? typeNameHint, bool addInterfacePrefix)
         {
             if (schema.Item != null)

src/NJsonSchema.CodeGeneration/ValueGeneratorBase.cs

@@ -48,12 +48,19 @@ protected ValueGeneratorBase(CodeGeneratorSettingsBase settings)
         /// <returns>The code.</returns>
         public virtual string? GetDefaultValue(JsonSchema schema, bool allowsNull, string targetType, string? typeNameHint, bool useSchemaDefault, TypeResolverBase typeResolver)
         {
+            var actualSchema = schema is JsonSchemaProperty ? ((JsonSchemaProperty)schema).ActualTypeSchema : schema.ActualSchema;
+
+            // Const values are always used as default values (they define the only valid value)
+            if (actualSchema.HasConstValue)
+            {
+                return GetConstantValue(actualSchema, targetType);
+            }
+
             if (schema.Default == null || !useSchemaDefault)
             {
                 return null;
             }
 
-            var actualSchema = schema is JsonSchemaProperty ? ((JsonSchemaProperty)schema).ActualTypeSchema : schema.ActualSchema;
             if (actualSchema.IsEnumeration && !actualSchema.Type.IsObject() && actualSchema.Type != JsonObjectType.None)
             {
                 return GetEnumDefaultValue(schema, actualSchema, typeNameHint, typeResolver);
@@ -114,6 +121,31 @@ protected string GetDefaultAsStringLiteral(JsonSchema schema)
             return ConversionUtilities.ConvertToStringLiteral(schema.Default?.ToString() ?? string.Empty, "\"", "\"");
         }
 
+        /// <summary>Gets the constant value code for a schema with a const value.</summary>
+        /// <param name="schema">The schema with a const value.</param>
+        /// <param name="targetType">The target type name.</param>
+        /// <returns>The code representing the constant value, or null if not applicable.</returns>
+        protected virtual string? GetConstantValue(JsonSchema schema, string targetType)
+        {
+            if (!schema.HasConstValue)
+            {
+                return null;
+            }
+
+            var constType = schema.ConstValueType;
+            if (constType.IsBoolean())
+            {
+                return schema.Const!.ToString()!.ToLowerInvariant();
+            }
+
+            if (constType.IsInteger() || constType.IsNumber())
+            {
+                return ConvertToNumberToStringCore(schema.Const!);
+            }
+
+            return ConversionUtilities.ConvertToStringLiteral(schema.Const!.ToString() ?? string.Empty, "\"", "\"");
+        }
+
         /// <summary>Converts a number to its string representation.</summary>
         /// <param name="value">The value.</param>
         /// <returns>The string.</returns>

src/NJsonSchema.Tests/Validation/ConstValidationTests.cs

@@ -0,0 +1,101 @@
+using NJsonSchema.Validation;
+
+namespace NJsonSchema.Tests.Validation
+{
+    public class ConstValidationTests
+    {
+        [Fact]
+        public async Task When_const_is_defined_and_value_matches_then_validation_succeeds()
+        {
+            // Arrange
+            var json = @"{ ""const"": ""person"" }";
+            var schema = await JsonSchema.FromJsonAsync(json);
+
+            // Act
+            var errors = schema.Validate(@"""person""");
+
+            // Assert
+            Assert.Empty(errors);
+        }
+
+        [Fact]
+        public async Task When_const_is_defined_and_value_does_not_match_then_validation_fails()
+        {
+            // Arrange
+            var json = @"{ ""const"": ""person"" }";
+            var schema = await JsonSchema.FromJsonAsync(json);
+
+            // Act
+            var errors = schema.Validate(@"""other""");
+
+            // Assert
+            Assert.Single(errors);
+            Assert.Equal(ValidationErrorKind.ConstantValueMismatch, errors.First().Kind);
+        }
+
+        [Fact]
+        public async Task When_const_is_integer_and_value_matches_then_validation_succeeds()
+        {
+            // Arrange
+            var json = @"{ ""const"": 42 }";
+            var schema = await JsonSchema.FromJsonAsync(json);
+
+            // Act
+            var errors = schema.Validate("42");
+
+            // Assert
+            Assert.Empty(errors);
+        }
+
+        [Fact]
+        public async Task When_const_is_integer_and_value_does_not_match_then_validation_fails()
+        {
+            // Arrange
+            var json = @"{ ""const"": 42 }";
+            var schema = await JsonSchema.FromJsonAsync(json);
+
+            // Act
+            var errors = schema.Validate("99");
+
+            // Assert
+            Assert.Single(errors);
+            Assert.Equal(ValidationErrorKind.ConstantValueMismatch, errors.First().Kind);
+        }
+
+        [Fact]
+        public void When_schema_is_serialized_with_const_then_const_is_included()
+        {
+            // Arrange
+            var schema = new JsonSchema
+            {
+                Const = "person"
+            };
+
+            // Act
+            var json = schema.ToJson();
+
+            // Assert
+            Assert.Contains("\"const\"", json);
+            Assert.Contains("\"person\"", json);
+        }
+
+        [Fact]
+        public async Task When_schema_with_const_is_deserialized_then_const_is_populated()
+        {
+            // Arrange
+            var json = @"{
+                ""type"": ""object"",
+                ""properties"": {
+                    ""cmdType"": { ""const"": ""person"" }
+                }
+            }";
+
+            // Act
+            var schema = await JsonSchema.FromJsonAsync(json);
+
+            // Assert
+            Assert.True(schema.Properties["cmdType"].HasConstValue);
+            Assert.Equal("person", schema.Properties["cmdType"].Const?.ToString());
+        }
+    }
+}